1. Field of the Invention
The present invention relates to a thermal head which has a plurality of thermal resistors for printing and is mainly used for a facsimile and a thermal printer.
2. Description of the Prior Art
Conventionally, it is well known that a thermal head has the disadvantage that the print density is likely to be uneven depending upon the temperature, print cycle, heat history of the thermal resistors or the like. To unify the print density, there has been proposed a thermal head in which thermal resistors are controlled in drive current value and conducting period.
FIG. 16 is a block diagram showing a prior art embodiment of a thermal head driving device. A thermal head 1 has a thermal resistor 2, a driver IC 3 driving the thermal resistor 2 and a thermistor 13 sensing the temperature of the thermal head. An A/D converter 4 converts analog temperature data received from the thermistor 13 into digital temperature data to apply it to a microcomputer 7. In an address of a ROM table 10 determined based upon the digital temperature data, print cycle data, head history data and the like, the microcomputer 7 accesses the ROM table 10 to obtain drive pulse width data and applies the drive pulse width data to a pulse width control circuit 5. The pulse width control circuit 5 generates a drive pulse signal (STROBE signal) having its pulse width controlled. In this way, the print density by the thermal head 1 keeps uniform (see International Application No. PCT/US87/01663, for example).
However, such a conventional thermal head driving device must have a considerably large capacity of memory of a circuit for externally driving the thermal head and considerably large amount of control program of the microcomputer, and further requires an expensive A/D converter in view of the digital processing of the microcomputer. Additionally, with a control system of a drive pulse for a thermal resistor provided in the thermal head, there arises the problem that the size of the thermal head is considerably increased.